Conventionally, there has been provided an ultrasound sensor, which is configured to determine presence or absence of an object in a detection area, and measure a distance to the object, using a time period from transmission of ultrasound waves to reception of waves reflected by the object.
This type of ultrasound sensor includes a transmitter configured to transmit ultrasound waves, and a receiver configured to receive ultrasound waves. Alternately, there has been provided an ultrasound sensor which includes a transmitting/receiving device that is used for both a transmitter and a receiver. The ultrasound sensor including the transmitting/receiving device is configured to drive the transmitting/receiving device with a transmission signal to transmit ultrasound waves to a detection area, and configured to detect reflected waves through a reception signal that is output from the transmitting/receiving device that has received ultrasound waves. Therefore, it is possible to determine presence or absence of an object in the detection area and measure a distance to the object, through measuring a time period from driving of the transmitting/receiving device with the transmission signal to detection of the reflected waves through the reception signal output from the transmitting/receiving device.
Because the transmitting/receiving device is used for transmission and reception of ultrasound waves, part of the transmission signal may turn to a circuit for detecting the reception signal. Furthermore, because the transmitting/receiving device vibrates mechanically when being driven with the transmission signal, reverberation may occur. For this reason, there has been proposed a configuration of setting a time period for detecting the reflected waves, when a time in which the sensor is not influenced by the reverberation comes, after driving of the transmitting/receiving device with the transmission signal. By adopting this configuration, it is possible to detect the reflected waves without the influence of the transmission signal and the reverberation (e.g., see JP2010-181208A).
Incidentally, attenuation of the reflected waves to the transmitted ultrasound waves is more increased, as a distance to an object to be detected is longer. Therefore, if energy of ultrasound waves to be transmitted is constant, amplitude of the reception signal is more reduced as the distance to the object to be detected is longer. From this, an increase in amplitude of ultrasound waves to be transmitted, or enhancement of reception sensibility for ultrasound waves can be considered, in order to more increase a distance at which the sensor can detect an object.
However, the increase in the amplitude of ultrasound waves to be transmitted causes a time period, during which the sensor is influenced by the reverberation, to be longer. Accordingly, it may cause a problem that the distance at which the sensor can detect an object is more reduced on a short-distance side. On the other hand, the enhancement of the reception sensibility causes an increase in a possibility of receiving ultrasound waves other than the waves reflected by the object. As a result, an S/N ratio becomes deteriorated, and it may cause a problem that erroneous detection easily occurs.